I can't see a villain named Rita Repulsa, let alone one who whines and is so shrill (if they keep those aspects) going over well. She was a silly joke then, would be more so now.
I remember vividly when they brought Lord Zedd in. He was truly scary. Truly a villain. He made Rita look as silly as she was. Dare I say he was the best villain on any season. Having him from the get-go for the baddie is just a no brainer. Zedd has the potential to be a Darth Vader type bad guy who despite being bad is popular.
The problem with Zeed is underneath his sinister appearance, he never got close to beating the rangers. Season 1 Rita had more near successes than Season 2 Zedd. After a while (season 2) Zedd devolved in to the same pitfalls as Rita did.
While by herself Rita had more near successes and formed better plans than Zedd. Rita's monster's nearly beating Rangers:
From Season 1
Minotaur - "Teamwork"
Sphinx - "A Pressing Engagement"
Terror Toad - "Power Ranger Punks"
Knasty Knight - "Happy Birthday Zack"
Green With Evil 5 parter
Frankenstein - "Life's a Masquerade"
Mutitus - "Island of Illusion"
Lokar - "Island of Illusion" + "Doomsday"
Cyclops - "Green Candle"
Hatchasaurus and Cardiatron "Birds of a feather"
Dramole - Return of and Old Friend"
Cyclopsis - "Doomsday"
Commander Crayfish and the Mutant Rangers - "Mighty Morphin Mutants"
In season 2
It was Rita's plan to trap the rangers in the abandoned mansion with a fraction of their powers and pursued by monsters. It was Rita's plan to bring back the Green Ranger powers to counter the White Ranger by making a clone Tommy. Rita was the architect plot for "Best Man For The Job" which netted her control of the Tigerzord.
In Season 3
Rita set in motion the theft of the Falconzord by recruiting Kat and having her steal the Pink Crane coin. This neutralized the Ninja Megazord from being summoned. It also led to Kimberly nearly dying and having to be hospitalized. From an attack from the Tengas.
While Rita and Zedd were a duo in during the tail end of season 2 and all of season 3. Zedd never showed the same level of efficiency and competency that Rita did; independently or allied.
I think of Zedd's closest attempts to beat the Rangers were when he dealt the killing blow to the Green Ranger powers after a tedious arc, and White Light with the Nimrod monster. With regards to the green ranger powers; Rita had already effectively taken them away in season 1. After White Light Zedd sucked at his job hard. Even Sepentera in the 4 episodes (3 of which as a weapon against the rangers) he was used in season 2 were a waste. Always stepping on Tor and or losing power.
Zedd is all show but no pony.
The best villains in my opinion
Rita - MMPR Season 1
Astronema - In Space Season 6
Trakeena - Lost Galaxy Season 7
Ransik - Timeforce Season 9
Mesogog - Dino Thunder Season 12
Grumm - SPD Season 13
Thrax - Operation Overdrive Season 15 (cheating here but Once a Ranger was too good)
Venjix - RPM Season 17
Criteria to judge are menace to the rangers
Execution of strategy and plans
Generals and supporting characters
Did the villains work thematically in their respective season
How close the big bad came to reaching their goals